HomeMy WebLinkAboutNantucket Harbor Estuaries Project Executive Summary_201401231100042002Executive Summary 1
Massachusetts Estuaries Project
Linked Watershed-Embayment Model
to Determine Critical Nitrogen Loading
Thresholds for Nantucket Harbor
Nantucket, Massachusetts
Executive Summary
1. Background
This report presents the results generated from the implementation of the Massachusetts
Estuaries Project’s Linked Watershed-Embayment Approach to the Nantucket Harbor
embayment system, a coastal embayment of the Island of Nantucket within the Town of
Nantucket, Massachusetts. Analyses of the Nantucket Harbor embayment system was
performed to assist the Town with up-coming nitrogen management decisions associated with
the Towns’ current and future wastewater planning efforts, as well as wetland restoration,
anadromous fish runs, shell fishery, open-space, and harbor maintenance programs. As part of
the MEP approach, habitat assessment was conducted on the embayment based upon
available water quality monitoring data, historical changes in eelgrass distribution, time-series
water column oxygen measurements, and benthic community structure. Nitrogen loading
thresholds for use as goals for watershed nitrogen management are the major product of the
MEP effort. In this way, the MEP offers a science-based management approach to support the
Town of Nantucket resource planning and decision-making process. The primary products of
this effort are: (1) a current quantitative assessment of the nutrient related health of the
Nantucket Harbor embayment, (2) identification of all nitrogen sources (and their respective N
loads) to embayment waters, (3) nitrogen threshold levels for maintaining Massachusetts Water
Quality Standards within embayment waters, (4) analysis of watershed nitrogen loading
reduction to achieve the N threshold concentrations in embayment waters, and (5) a functional
calibrated and validated Linked Watershed-Embayment modeling tool that can be readily used
for evaluation of nitrogen management alternatives (to be developed by the Town) for the
restoration of the Nantucket Harbor embayment system.
Wastewater Planning: As increasing numbers of people occupy coastal watersheds, the
associated coastal waters receive increasing pollutant loads. Coastal embayments throughout
the Commonwealth of Massachusetts (and along the U.S. eastern seaboard) are becoming
nutrient enriched. The elevated nutrients levels are primarily related to the land use impacts
associated with the increasing population within the coastal zone over the past half-century.
The regional effects of both nutrient loading and bacterial contamination span the
spectrum from environmental to socio-economic impacts and have direct consequences to the
Massachusetts
Department of
Environmental
Protection
Executive Summary 2
culture, economy, and tax base of Massachusetts’s coastal communities. The primary nutrient
causing the increasing impairment of our coastal embayments is nitrogen, with its primary
sources being wastewater disposal, and nonpoint source runoff that carries nitrogen (e.g.
fertilizers) from a range of other sources. Nitrogen related water quality decline represents one
of the most serious threats to the ecological health of the nearshore coastal waters. Coastal
embayments, because of their shallow nature and large shoreline area, are generally the first
coastal systems to show the effect of nutrient pollution from terrestrial sources.
In particular, the Nantucket Harbor embayment system within the Town of Nantucket is at
risk of eutrophication (over enrichment) from enhanced nitrogen loads entering through
groundwater from the increasingly developed watershed to this coastal system. Eutrophication
is a process that occurs naturally and gradually over a period of tens or hundreds of years.
However, human-related (anthropogenic) sources of nitrogen may be introduced into
ecosystems at an accelerated rate that cannot be easily absorbed, resulting in a phenomenon
known as cultural eutrophication. In both marine and freshwater systems, cultural
eutrophication results in degraded water quality, adverse impacts to ecosystems, and limits on
the use of water resources.
The relatively pristine nature of Nantucket's nearshore and Harbor waters has historically
been a valuable asset to the island. However, concern over the potential degradation of Harbor
water quality began to arise, which resulted in monitoring, scientific investigations and
management planning which continues to this day. Nantucket Harbor is one of the largest
enclosed bays in southeastern Massachusetts and one of the few with a relatively high water
quality capable of supporting significant high quality ecological habitats, such as eelgrass beds,
and sustain a scallop fishery. Ironically, it is the pristine nature of this system which may
indirectly threaten its ecological health as the coastal waters throughout Southeastern New
England become increasingly degraded and the pressure for access and development of
remaining high quality environments increases. The Town of Nantucket and work groups have
long ago recognized that a rigorous scientific approach yielding site-specific nitrogen loading
targets was required for decision-making, alternatives analysis and ultimately, habitat protection.
The completion of this multi-step process has taken place under the programmatic umbrella of
the Massachusetts Estuaries Project, which is a partnership effort between all MEP
collaborators and the Town. The modeling tools developed as part of this program provide the
quantitative information necessary for the Towns’ nutrient management groups to predict the
impacts on water quality from a variety of proposed management scenarios.
Nitrogen Loading Thresholds and Watershed Nitrogen Management: Realizing the
need for scientifically defensible management tools has resulted in a focus on determining the
aquatic system’s assimilative capacity for nitrogen. The highest-level approach is to directly link
the watershed nitrogen inputs with embayment hydrodynamics to produce water quality results
that can be validated by water quality monitoring programs. This approach when linked to state-
of-the-art habitat assessments yields accurate determination of the “allowable N concentration
increase” or “threshold nitrogen concentration”. These determined nitrogen concentrations are
then directly relatable to the watershed nitrogen loading, which also accounts for the spatial
distribution of the nitrogen sources, not just the total load. As such, changes in nitrogen load
from differing parts of the embayment watershed can be evaluated relative to the degree to
which those load changes drive embayment water column nitrogen concentrations toward the
“threshold” for the embayment system. To increase certainty, the “Linked” Model is
independently calibrated and validated for each embayment.
Executive Summary 3
Massachusetts Estuaries Project Approach: The Massachusetts Department of
Environmental Protection (DEP), the University of Massachusetts – Dartmouth School of Marine
Science and Technology (SMAST), and others including the Cape Cod Commission (CCC)
have undertaken the task of providing a quantitative tool to communities throughout
southeastern Massachusetts (the Linked Watershed-Embayment Management Model) for
nutrient management in their coastal embayment systems. Ultimately, use of the Linked
Watershed-Embayment Management Model tool by municipalities in the region results in
effective screening of nitrogen reduction approaches and eventual restoration and protection of
valuable coastal resources. The MEP provides technical guidance in support of policies on
nitrogen loading to embayments, wastewater management decisions, and establishment of
nitrogen Total Maximum Daily Loads (TMDLs). A TMDL represents the greatest amount of a
pollutant that a waterbody can accept and still meet water quality standards for protecting public
health and maintaining the designated beneficial uses of those waters for drinking, swimming,
recreation and fishing. The MEP modeling approach assesses available options for meeting
selected nitrogen goals that are protective of embayment health and achieve water quality
standards.
The core of the Massachusetts Estuaries Project analytical method is the Linked
Watershed-Embayment Management Modeling Approach, which links watershed inputs with
embayment circulation and nitrogen characteristics.
The Linked Model builds on well-accepted basic watershed nitrogen loading approaches
such as those used in the Buzzards Bay Project, the CCC models, and other relevant models.
However, the Linked Model differs from other nitrogen management models in that it:
• requires site-specific measurements within each watershed and embayment;
• uses realistic “best-estimates” of nitrogen loads from each land-use (as opposed to loads
with built-in “safety factors” like Title 5 design loads);
• spatially distributes the watershed nitrogen loading to the embayment;
• accounts for nitrogen attenuation during transport to the embayment;
• includes a 2D or 3D embayment circulation model depending on embayment structure;
• accounts for basin structure, tidal variations, and dispersion within the embayment;
• includes nitrogen regenerated within the embayment;
• is validated by both independent hydrodynamic, nitrogen concentration, and ecological data;
• is calibrated and validated with field data prior to generation of “what if” scenarios.
The Linked Model Approach’s greatest assets are its ability to be clearly calibrated and
validated, and its utility as a management tool for testing “what if” scenarios for evaluating
watershed nitrogen management options.
For a comprehensive description of the Linked Model, please refer to the Full Report:
Nitrogen Modeling to Support Watershed Management: Comparison of Approaches and
Sensitivity Analysis, available for download at http://www.state.ma.us/dep/smerp/smerp.htm. A
more basic discussion of the Linked Model is also provided in Appendix F of the Massachusetts
Estuaries Project Embayment Restoration Guidance for Implementation Strategies, available for
download at http://www.state.ma.us/dep/smerp/smerp.htm. The Linked Model suggests which
management solutions will adequately protect or restore embayment water quality by enabling
towns to test specific management scenarios and weigh the resulting water quality impact
against the cost of that approach. In addition to the management scenarios modeled for this
report, the Linked Model can be used to evaluate additional management scenarios and may be
Executive Summary 4
updated to reflect future changes in land-use within an embayment watershed or changing
embayment characteristics. In addition, since the Model uses a holistic approach (the entire
watershed, embayment and tidal source waters), it can be used to evaluate all projects as they
relate directly or indirectly to water quality conditions within its geographic boundaries. Unlike
many approaches, the Linked Model accounts for nutrient sources, attenuation, and recycling
and variations in tidal hydrodynamics and accommodates the spatial distribution of these
processes. For an overview of several management scenarios that may be employed to restore
embayment water quality, see Massachusetts Estuaries Project Embayment Restoration
Guidance for Implementation Strategies, available for download at
http://www.state.ma.us/dep/smerp/smerp.htm.
Application of MEP Approach: The Linked Model was applied to the Nantucket Harbor
embayment system by using site-specific data collected by the MEP and water quality data from
the Water Quality Monitoring Program conducted by the Nantucket Marine Department, with
technical guidance from the Coastal Systems Program at SMAST (see Section II). Evaluation
of upland nitrogen loading was conducted by the MEP. Estuaries Project staff obtained digital
parcel and tax assessors data from the Town of Nantucket Geographic Information Systems
Department, watershed specific water use data from the Wannacomet Water Company (WWC)
and watershed boundaries adopted by the town as the Harbor Watershed Protection District
(http://www.nantucket-ma.gov). During the development of the Nantucket Water Resources
Management Plan, an island-wide groundwater mapping project, using many of the USGS wells
on the Island, was completed to characterize the water table configuration of Nantucket
(Horsley, Whittan, Hegeman, 1990). Estuary watershed delineations completed in areas with
relatively transmissive sand and gravel deposits, like most of Cape Cod and the Islands, have
shown that watershed boundaries are usually better defined by elevation of the groundwater
and its direction of flow, rather than by land surface topography (Cambareri and Eichner 1998,
Millham and Howes 1994a,b). This approach was used by Horsley, Whittan and Hegeman, Inc.
(HWH) to complete a watershed delineation for Nantucket Harbor (Section III); this watershed
delineation was been largely confirmed by subsequent water table characterizations (e.g.,
Lurbano, 2001, Gardner and Vogel, 2005). MEP staff compared the HWH Harbor watershed to
a 2004 aerial base map. This comparison found some slight discrepancies likely based on a
better characterization of the shoreline; changes were made based on best professional
judgment and watershed/water table characterization experience in similar geologic settings
The land-use data obtained from the Town was used to determine watershed nitrogen
loads within the Nantucket Harbor embayment system and each of the systems sub-
embayments as appropriate (current and build-out loads are summarized in Section IV). Water
quality within a sub-embayment is the integration of nitrogen loads with the site-specific
estuarine circulation. Therefore, water quality modeling of this tidally influenced estuary
included a thorough evaluation of the hydrodynamics of the estuarine system. Estuarine
hydrodynamics control a variety of coastal processes including tidal flushing, pollutant
dispersion, tidal currents, sedimentation, erosion, and water levels. Once the hydrodynamics of
the system was quantified, transport of nitrogen was evaluated from tidal current information
developed by the numerical models.
A two-dimensional depth-averaged hydrodynamic model based upon the tidal currents
and water elevations was employed for the Nantucket Harbor embayment system. Once the
hydrodynamic properties of the estuarine system were computed, two-dimensional water quality
model simulations were used to predict the dispersion of the nitrogen at current loading rates.
Using standard dispersion relationships for estuarine systems of this type, the water quality
model and the hydrodynamic model was then integrated in order to generate estimates
Executive Summary 5
regarding the spread of total nitrogen from the site-specific hydrodynamic properties. The
distributions of nitrogen loads from watershed sources were determined from land-use analysis.
Boundary nutrient concentrations in Nantucket Sound source waters were taken from water
quality monitoring data. Measurements of current salinity distributions throughout the estuarine
waters of the Nantucket Harbor embayment system was used to calibrate the water quality
model, with validation using measured nitrogen concentrations (under existing loading
conditions). The underlying hydrodynamic model was calibrated and validated independently
using water elevations measured in time series throughout the embayments.
MEP Nitrogen Thresholds Analysis: The threshold nitrogen level for an embayment
represents the average water column concentration of nitrogen that will support the habitat
quality being sought. The water column nitrogen level is ultimately controlled by the watershed
nitrogen load and the nitrogen concentration in the inflowing tidal waters (boundary condition).
The water column nitrogen concentration is modified by the extent of sediment regeneration.
Threshold nitrogen levels for the embayment systems in this study were developed to restore or
maintain SA waters or high habitat quality. High habitat quality was defined as supportive of
eelgrass and infaunal communities. Dissolved oxygen and chlorophyll a were also considered
in the assessment.
The nitrogen thresholds developed in Section VIII-2 were used to determine the amount of
total nitrogen mass loading reduction required for restoration of eelgrass and infaunal habitats in
the Nantucket Harbor system. Tidally averaged total nitrogen thresholds derived in Section
VIII.1 were used to adjust the calibrated constituent transport model developed in Section VI.
Watershed nitrogen loads were sequentially lowered, using reductions in septic effluent
discharges only, until the nitrogen levels reached the threshold level at the sentinel station
chosen for the Nantucket Harbor system. It is important to note that load reductions can be
produced by reduction of any or all sources or by increasing the natural attenuation of nitrogen
within the freshwater systems to the embayment. The load reductions presented below
represent only one of a suite of potential reduction approaches that need to be evaluated by the
community. The presentation is to establish the general degree and spatial pattern of reduction
that will be required for protection/restoration of this nitrogen threatened embayment.
The Massachusetts Estuaries Project’s thresholds analysis, as presented in this technical
report, provides the site-specific nitrogen reduction guidelines for nitrogen management of the
Nantucket Harbor embayment system in the Town of Nantucket. Future water quality modeling
scenarios should be run which incorporate the spectrum of strategies that result in nitrogen
loading reduction to the embayment. The MEP analysis has initially focused upon nitrogen
loads from on-site septic systems as a test of the potential for achieving the level of total
nitrogen reduction for restoration of the embayment system. The concept was that since septic
system nitrogen loads generally represent 28% - 53% of the controllable watershed load to the
Nantucket Harbor embayment system and are more manageable than other of the nitrogen
sources, the ability to achieve needed reductions through this source is a good gauge of the
feasibility for protection/restoration of the system.
2. Problem Assessment (Current Conditions)
A habitat assessment was conducted throughout the Nantucket Harbor system based
upon available water quality monitoring data, historical changes in eelgrass distribution, time-
series water column oxygen measurements, and benthic community structure. At present, the
Nantucket Harbor System is showing variations in nitrogen enrichment among its 4 principal
component basins. The inner basins of Head of the Harbor and Polpis Harbor are nitrogen
Executive Summary 6
enriched over Quaise and the Town basins. Although the component basins of the Nantucket
Harbor System are clearly enriched in nitrogen over the adjacent Nantucket Sound waters, the
enrichment is relatively small, generally <0.100 mg L-1 (see Chapter VI).
The effect of nitrogen enrichment is to cause oxygen depletion; however, with increased
phytoplankton (or epibenthic algae) production, oxygen levels will rise in daylight to above
atmospheric equilibration levels in shallow systems (generally ~7-8 mg L-1 at the mooring sites).
Overall, oxygen within the Harbor bottom waters appears to remain at ecologically healthy
levels, except for periodic oxygen depletion within the deepest portions of the Quaise and
Wauwinet basins. However, as there were some oxygen depletions below 5 mg L-1 in the main
basins (although infrequent), it appears that the system is at or just beyond it ability to assimilate
additional nitrogen/organic matter.
Within the highly flushed and generally well mixed waters of the lower basins of Nantucket
Harbor, bottom waters were well oxygenated (>6mg L-1). The few excursions below 6 mg L-1
were isolated events, rather than a prolonged depletion such as generally associated with a
phytoplankton bloom. However, these variations were small and overall the oxygen conditions
are consistent with the observations of healthy infaunal and eelgrass communities. While Polpis
Harbor also exhibited well oxygenated conditions, larger diurnal variations were recorded than
in the outer basins. The higher diurnal fluctuations indicate waters supporting higher
phytoplankton biomass. Quaise basin showed both significant diurnal oxygen fluctuations and
an overall oxygen decline, although not to levels of high stress. There was a single "event" of a
few days when each night oxygen levels reached 4 mg/L, but returned to ~5 mg L-1 each
following day. Since the meter was located deeper within the basin (~6 m), oxygen levels
throughout most of the basin area were almost certainly higher given their shallower depths,
only in the "deep hole" was oxygen depletion likely greater. Assessing oxygen conditions within
the Quaise basin indicates generally non-stressful oxygen levels, except for the deep basin.
However, it is likely that the presence of the deep hole (~30') creates a geomorphological
(natural) cause of the low dissolved oxygen. Head of the Harbor showed generally high oxygen
levels. As in the Quaise basin, the meter was deeper in the basin and observed oxygen
depletions were greater than experienced by bottom waters throughout most of the basin area.
The oxygen conditions are consistent with the observed distribution of habitat quality throughout
the Harbor System, with the deep waters showing oxygen depletion, but with oxygen levels
generally supportive of a high habitat quality for infauna. However, since the system does show
oxygen levels less than full atmospheric saturation, additional organic matter loads, (e.g.
through nitrogen inputs) will likely increase the magnitude and frequency of the oxygen declines,
again indicating a system at or just beyond its nitrogen assimilative capacity (nitrogen threshold)
Based upon all available data it appears that eelgrass is presently a widespread critical
habitat within the Nantucket Harbor System. The present distribution of eelgrass results from
recolonization of the Harbor from its loss in the 1930's. A map of eelgrass from the 1940's
"shows it to be primarily confined to parts of the Jetties and Horse shed at the Harbor entrance
(Kelley 1989). Kelley (1989) concluded that from the 1960's to 1989, "eelgrass distribution has
been relatively stable in Nantucket Harbor...". However, it is clear that eelgrass beds have been
lost from this System. Both the MassDEP analysis and the direct observations of Kelley in 1989
indicated that there has been measurable eelgrass loss. The primary locations are within Head
of the Harbor and East Polpis Harbor. The other major region experiencing gradual losses, the
marginal areas of Head of the Harbor, is supported by both Kelley (1989) and the MassDEP
survey data. This larger areal loss appears to be gradual and occurring primarily in the least
well flushed areas of this basin (note the counterclockwise circulation). Eelgrass loss has also
been noted to the west of Pocomo, which was observed in the 1980 surveys and more recently
Executive Summary 7
in changes from 1995-2001. It is important to note that the eelgrass bed loss is both from the
shallow area of the upper and mid regions of Head of the Harbor (<8' depth) and from the
"deeper" areas (8'-12') in the lower reach and from the shallow east basin of Polpis. The data
indicate that that on the order of 1000 acres of eelgrass habitat within the Nantucket Harbor
System is impaired.
It is important to note that the nitrogen levels throughout the Nantucket Harbor System
remain relatively low, consistent with the observed oxygen conditions, lack of macroalgae and
chlorophyll a levels. However, due to the water depth in the Harbor, it is possible that vertical
and horizontal mixing rates appear to have resulted in a decline in eelgrass bed coverage from
the deeper areas and more enclosed basin areas.
Macro-algal abundance within the Harbor surveyed in 1994 (Harbor Study 1997) was
typical of a relatively healthy environment. Algal cover was highest on the Nantucket Sound
side between the points of Coatue (Figure VII-10). The highest concentrations of macro-algae
were consistent with the circulation patterns associated with the cusps of land present around
the Harbor edge. It also appears that the macro-algal accumulations are not related to
terrestrial nitrogen inputs, since the "island" side of the Harbor, which dominates the land based
loadings, had lower algal accumulations than Coatue. The absence of macroalgal
accumulations and drift algae is consistent with the generally low nitrogen levels throughout this
System and the relatively low watershed nitrogen input.
The infaunal data clearly show that the lower basins and shallower areas (<12') of the
main Harbor basins generally support high quality infaunal habitat. The lowermost basin (Town)
exhibited a dense, highly diverse and relatively evenly distributed community, with some
variation. The shallower margins of both Quaise and Head of the Harbor were only slightly less
diverse than areas nearer the tidal inlet, but were clearly of high quality. This is further
evidenced by the growth of epibenthic scallops in these areas. Within the main Harbor basins,
only the deep "holes" showed reduced numbers of species and individuals and organic
enrichment indicators. This indication of moderate to poor habitat in these deep regions is
consistent with previous analyses and supported by the observed accumulations of organic
detritus in these natural depositional areas. It is unlikely that management of nitrogen loading
will be able to create significant improvement within these deep basin regions and it is likely that
these areas have been "stressed" by natural processes for a long time.
Overall, the MEP system-wide infaunal survey found higher numbers of species and
individuals in communities that were generally more diverse and evenly distributed than the
other 20 embayments examined to date by the MEP in southeastern Massachusetts. This is
consistent with the relatively low tidally averaged nitrogen levels within the system, <0.40 mg N
L-1 and generally 0.285-0.361 mg N L-1.
3. Conclusions of the Analysis
The threshold nitrogen level for an embayment represents the average watercolumn
concentration of nitrogen that will support the habitat quality being sought. The watercolumn
nitrogen level is ultimately controlled by the integration of the watershed nitrogen load, the
nitrogen concentration in the inflowing tidal waters (boundary condition) and dilution and
flushing via tidal flows. The water column nitrogen concentration is modified by the extent of
sediment regeneration and by direct atmospheric deposition.
Executive Summary 8
Threshold nitrogen levels for this embayment system were developed to restore or
maintain SA waters or high habitat quality. In this system, high habitat quality was defined as
supportive of eelgrass and supportive of diverse benthic animal communities. Dissolved oxygen
and chlorophyll a were also considered in the assessment.
Watershed nitrogen loads (Tables ES-1 and ES-2) for the Town of Nantucket, Nantucket
Harbor embayment system was comprised primarily of runoff from impervious surfaces,
fertilizers and wastewater nitrogen. Land-use and wastewater analysis found that generally
about 28% - 53% of the controllable watershed nitrogen load to the embayment was from
wastewater.
A major finding of the MEP clearly indicates that a single total nitrogen threshold can not
be applied to Massachusetts’ estuaries, based upon the results of the Great, Green and
Bournes Pond Systems, Popponesset Bay System, the Hamblin / Jehu Pond / Quashnet River
analysis in eastern Waquoit Bay, the analysis of the adjacent Rushy Marsh system and the
Pleasant Bay and Nantucket Sound embayments associated with the Town of Chatham. This is
almost certainly going to be true for the other embayments within the MEP area, as well.
The threshold nitrogen levels for the Nantucket Harbor embayment system in Nantucket
were determined as follows:
Nantucket Harbor Threshold Nitrogen Concentrations
• Following the MEP protocol, the restoration target for the Nantucket Harbor system
should reflect both recent pre-degradation habitat quality and be reasonably achievable.
Determination of the critical nitrogen threshold for maintaining high quality habitat within
the Nantucket Harbor Estuarine System is based primarily upon the nutrient and oxygen
levels, temporal trends in eelgrass distribution and current benthic community indicators.
The Nantucket Harbor System is presently supportive of infaunal habitat throughout its
main basins, but is clearly impaired by nitrogen enrichment within the Head of the
Harbor basin and in the eastern basin of Polpis Harbor, based upon eelgrass losses.
Given the documented importance of eelgrass habitat to these basins and the
demonstrable loss of eelgrass that were supported, eelgrass restoration in these basins
was set as the primary nitrogen management goal for the overall System. Due to the
semi-isolated nature of Polpis Harbor from Nantucket Harbor, it is necessary to establish
2 sentinel stations for eelgrass, one in the Head of the Harbor and one in the east basin
of Polpis Harbor (e.g. where eelgrass had been observed in 1951-1989).
• It is important to note that the nitrogen levels throughout the Nantucket Harbor System
remain relatively low, consistent with the oxygen conditions, lack of macroalgae and
chlorophyll a levels. However, the water depth of the Harbor and possibly vertical and
horizontal mixing rates appear to have resulted in a decline in eelgrass bed coverage
from the deeper areas and more enclosed basin areas. While eelgrass was only
recently lost from the east basin of Polpis Harbor, it is presently absent at a tidally
average total nitrogen (TN) level of 0.361 mg N L-1. Loss at this nitrogen level is
consistent with observed losses in West Falmouth Harbor above 0.350 mg N L-1,
however, given the shallower depth of Polpis Harbor, it is likely that it is just slightly
above its threshold level at present. Similarly, tidally averaged levels in the lower reach
of Head of the Harbor (0.340-0.353) and mid and upper reach (0.390 mg N L-1) also
suggest that the recent bed losses are from a recent exceedance of the supportive
nitrogen threshold. Given all of the factors discussed above and the similarity of Head of
Executive Summary 9
the Harbor to conditions in West Falmouth and Phinneys Harbors and its present
nitrogen levels, a nitrogen threshold of 0.350 mg N L-1 was determined to be supportive
of eelgrass habitat in this system. This threshold should also support eelgrass in the
shallower regions as well. As the east basin of Polpis Harbor has only recently lost its
eelgrass and is presently 0.361 mg N L-1, but has shallower waters than Head of the
Harbor, only a slight reduction over present levels appears to be needed to support
eelgrass habitat. Clearly the threshold must be lower than the present 0.361 mg N L-1
and higher than that for Head of the Harbor (0.350 mg N L-1). Therefore, a threshold of
0.355 mg N L-1 was set for the sentinel station in Polpis Harbor. It should be noted that
the Polpis Harbor threshold is well constrained by the available data, but is at the limits
of the sensitivity of the MEP approach.
It is important to note that the analysis of future nitrogen loading to the Nantucket
Harbor estuarine system focuses upon additional shifts in land-use from forest/grasslands to
residential and commercial development. However, the MEP analysis indicates that
increases in nitrogen loading can occur under present land-uses, due to shifts in occupancy,
shifts from seasonal to year-round usage and increasing use of fertilizers. Therefore,
watershed-estuarine nitrogen management must include management approaches to
prevent increased nitrogen loading from both shifts in land-uses (new sources) and from
loading increases of current land-uses. The overarching conclusion of the MEP analysis of
the Nantucket Harbor estuarine system is that protection/restoration will necessitate a
reduction in the present (2003) nitrogen inputs and management options to negate
additional future nitrogen inputs.
Executive Summary 10 Table ES-1. Existing total and sub-embayment nitrogen loads to the estuarine waters of the Nantucket Harbor system, observed nitrogen concentrations, and sentinel system threshold nitrogen concentrations. Loads to estuarine waters of the Nantucket Harbor system include both upper watershed regions contributing to the major surface water inputs. Sub-embayments Natural Background Watershed Load 1 (kg/day) Present Land Use Load 2 (kg/day) Present Septic System Load (kg/day) Present WWTF Load 3 (kg/day) Present Watershed Load 4 (kg/day) Direct Atmospheric Deposition 5 (kg/day) Present Net Benthic Flux (kg/day) Present Total Load 6 (kg/day) Observed TN Conc. 7 (mg/L) Threshold TN Conc. 8 (mg/L) NANTUCKET HARBOR SYSTEM Head of the Harbor 0.526 1.152 0.705 0.000 1.858 22.239 -17.211 6.886 0.34-0.41 -- Polpis Harbor 1.836 3.094 0.435 0.000 3.529 2.190 27.441 33.160 0.36-0.39 -- Quaise Basin 0.896 1.731 0.392 0.000 2.123 20.126 43.896 66.145 0.34 -- Town Basin 1.321 10.708 5.194 0.000 15.901 13.888 -2.793 26.997 0.30-0.34 -- Nantucket Harbor System Total 4.578 16.685 6.726 0.000 23.411 58.443 51.333 133.187 0.30-0.41 0.355 1 assumes entire watershed is forested (i.e., no anthropogenic sources) 2 composed of non-wastewater loads, e.g. fertilizer and runoff and natural surfaces and atmospheric deposition to lakes 3 existing unattenuated wastewater treatment facility discharges to groundwater 4 composed of combined natural background, fertilizer, runoff, and septic system loadings 5 atmospheric deposition to embayment surface only. 6 composed of natural background, fertilizer, runoff, septic system atmospheric deposition and benthic flux loadings 7 average of data collected between 1988 and 2005, ranges show the upper to lower regions (highest-lowest) of a sub-embayment. 8 Eel grass threshold for sentinel site located at Polpis Harbor.
Executive Summary 11 Table ES-2. Present Watershed Loads, Thresholds Loads, and the percent reductions necessary to achieve the Thresholds Loads for the Nantucket Harbor system. Two threshold scenarios are presented for the Harbor: Scenario A (N1) with 100% removal of septic load from the Town watershed together with 80% removal of anthropogenic watershed loads (septic, fertilizer and non-pervious surfaces) from the remaining three Harbor watersheds; and Scenario B (N2) with the removal of 100% of septic loads from all four of the Harbor Watersheds. Sub-embayments Present Watershed Load 1 (kg/day) Target Threshold Watershed Load 2 (kg/day) Direct Atmospheric Deposition (kg/day) Benthic Flux Net 3 (kg/day) TMDL 4 (kg/day) Percent watershed reductions needed to achieve threshold load levels NANTUCKET HARBOR SYSTEM Head of the Harbor 1.858 N1: 0.792 N2: 1.153 22.239 N1: -16.795 N2: -17.182 N1: 6.235 N2: 6.210 N1: -57.4% N2: -37.9% Polpis Harbor 3.529 N1: 2.175 N2: 3.093 2.190 N1: 26.450 N2: 26.655 N1: 30.816 N2: 31.939 N1: -38.4% N2: -12.3% Quaise Basin 2.123 N1: 1.140 N2: 1.732 20.126 N1: 43.010 N2: 42.885 N1: 64.276 N2: 64.743 N1: -46.3% N2: -18.5% Town Basin 15.901 N1: 10.707 N2: 10.707 13.888 N1: -2.892 N2: -2.892 N1: 21.702 N2: 21.702 N1: -32.7% N2: -32.7% Nantucket Harbor System Total 23.411 N1: 14.814 N2: 16.685 58.443 N1: 49.772 N2: 49.466 N1: 123.029 N2: 124.594 N1: -36.7% N2: -28.7% (1) Composed of combined natural background, fertilizer, runoff, and septic system loadings. (2) Target threshold watershed load is the load from the watershed needed to meet the embayment threshold concentration identified in Table ES-1. (3) Projected future flux (present rates reduced approximately proportional to watershed load reductions). (4) Sum of target threshold watershed load, atmospheric deposition load, and benthic flux load.